The hunt for the Ipswich serial killer spread across Europe last night as detectives revealed the murderer used only his bare hands to kill his five victims.

In the latest twist in one of Britain's biggest police inquiries, Detective Chief Superintendent Stewart Gull said they were no longer 'looking for a murder weapon', strengthening fears the women were in a state of drug-induced unconsciousness before they were killed.

This news coincided with an abrupt escalation of the investigation with confirmation that officers across the Channel have joined the hunt. Gull said he would not 'second guess' what the killer might do next.

The officer, who is leading the inquiry into Britain's most prolific serial killer, said formal contact had been made with Interpol, the organisation that co-ordinates criminal investigation across Europe and the world.

One theory is that the murderer may have passed through the port of Felixstowe, near Ipswich, or is an itinerant who has returned to mainland Europe.

The latest development came as police revealed yesterday that 24-year-old victim Anneli Alderton was three months pregnant when she was murdered.

They released new closed-circuit TV footage tracing the last sighting of her as she caught the 5.53pm train from Harwich to Colchester a fortnight ago. Anneli had been to visit her five-year-old son. The images provide for the first time a clear indication of what Anneli was wearing when she disappeared: a black jacket with a fur-trimmed hood, blue jeans, a grey top and white footwear. She was carrying a shiny nylon shoulder bag.

Officers don't know where Anneli met the murderer. Gull said it 'was possible' she was abducted and murdered outside Ipswich, which would challenge the assumption that all the victims met their killer in the town's red-light district.

In a separate twist, police want to track down a man who violently attacked two prostitutes, including one of the victims, just weeks before Tania went missing. A 42-year-old prostitute from Ipswich told The Observer she and Annette had discussed whether their attacker could have been responsible for the first two murders, of Gemma and Tania. Soon after, Annette went missing. 'We will be looking to speak to the individuals involved,' said Gull.

One of the central lines of inquiry emerging is finding answers to how the bodies of the victims, all found naked in fields and streams around Ipswich, bore no marks of a struggle. Police want to determine if all five victims were drugged, either by poison or a heavy dosage of crack cocaine, or heroin that may have been used as a lure.

All five were addicted to hard drugs and used prostitution to pay for their habit. It is known Paula was carrying scissors to defend herself, yet she seemingly failed to put up a fight. Police said yesterday that toxicology tests would not be ready for at least four weeks.

Tracking down 50 'individuals of interest' is another priority for Operation Sumac this week. The driver of a blue BMW is among those questioned, although police said they have not ruled him out of further interviews. Another person of interest is a man who tried to develop images of nearly 300 women in Ipswich, all wearing revealing clothing. The photographs appeared to be taken without subjects' knowledge. When the man tried to get them developed at a Tesco store in Copdock, near where Tania's body was found, police were called. He is likely to be questioned.

Around 9,000 calls have been made to the inquiry hotline with more than 30 forces - including Northern Ireland - involved. A reconstruction of the last movements of one or some of the prostitutes is likely to be announced. The spokesman said no decision has yet been made on what it would involve or when it would take place.